Generally, radiographic images such as X-ray images have been commonlly utilized for diagnoses of condition of a patient at medical scenes. In particular, radiographic images by an intensifying screen-film system, as a result of achievement of a high sensitivity and a high image quality during the long improvement history, are still utilized at medial scenes all over the world as an image pick-up system provided with the both of high reliability and superior cost performance.
However, the image information is so-called analogue image information, and it is impossible to perform free image processing and image transmission in a moment as with digital image information which has been ever developing in recent years.
Therefore, in recent years, a radiographic image detector system such as computed radiography (CR) and flat-panel type radiation detector (FPD) has come to be in practical use. Since these can directly obtain a digital radiographic image and directly display the image on an image display device such as a cathode ray tube and a liquid crystal panel, there is not necessarily required image formation on photographic film. As a result, these digital X-ray image detector systems have decreased necessity of image formation by silver salt photography and significantly improved convenience of diagnostic works at hospitals and clinics.
CR has come to be in practical use in medical scenes at present as one of digital technologies of X-ray images. However, the sharpness is not sufficient nor the spatial resolution is, and CR has not achieved an image quality of a screen-film system. In addition, flat plate X-ray detector system (FPD) employing thin film transistor (TFT), described in such as “Amorphous Semiconductor Usher in Digital X-ray Imaging” by John Rawlands, Physics Today, 1997 Nov., p. 24, and “Development of a High Resolution, Active Matrix, Flat-Panel Imager with Enhanced Fill Factor” by L. I. Anthonuk, SPIE, 1997, vol. 32, p. 2, as a further new digital X-ray image technology has been developed.
It is a feature that such a FPD is smaller in size than a CR and is superior in image quality of image pick-up at a high dose. However, on the other hand a FPD produced a problem such that an SN ratio is decreased at image pick-up at a low dose, accompanied with a insufficient image quality, because of electric noise caused by providing a TFT as well as a circuit.
In such a FPD, utilized is a scintillator plate, which is prepared by employing an X-ray fluorescent material provided with a property of emitting via radiation to convert radiation into visible light, and it is necessary to utilize a scintillator plate having a high emission efficiency to improve an SN ratio in image pick-up at a low dose. Generally, the emission efficiency of a scintillator plate is determined by a thickness of a fluorescent layer and an X-ray absorption coefficient of a fluorescent material, however, the thicker the fluorescent layer thickness, the more sharpness is decreased due to scattering of emission light in a fluorescent layer.
Cesium iodide (CsI) here has a relatively high conversion ratio from X-ray to visible light and the fluorescent material is possible to be formed into a columnar crystal structure by vacuum evaporation. An optical guide effect, in which luminescence within the crystals that is emitted from the side surfaces of the columnar crystals is reduced, can be obtained by providing a phosphor composed of columnar crystals. That is, the scattering of luminescence as well as the sharpness drop is suppressed. It is accordingly possible that the thickness of a phosphor layer is made thicker without degrading sharpness. A CsI columnar crystal diameter is generally 3-20 μm, and the thinner the columnar crystal diameter is, the higher the optical guide effect is obtained, resulting in enhanced sharpness.
Incidentally, it is strongly desired to uniform an image quality distribution within a detector plane, since the detection area of FPD is large. In other words, it is desired to uniform the distribution within the plane concerning various phosphor properties in order to improve the image quality, since the distribution within the plane depends largely on an image distribution. Specifically, it is also desired to uniform the phosphor columnar crystal diameter within the detector plane since sharpness depends largely on a phosphor columnar crystal diameter.
Therefore, as described in Patent Document 1, known is a scintillator plate for radiation in which the phosphor columnar crystal diameter becomes larger from the center of a phosphor layer toward the periphery. According to such the scintillator plate for radiation, no damage is to occur during adhesion employing a roller and a sensor panel equipped with a photoelectric transducer section when phosphor is strengthened by making phosphor columnar crystals at the periphery thicker, whereby an image with no blur together with no damage of phosphor at the production stage can be obtained.
Also known is a scintillator plate which is fixed by covering a phosphor layer with films after preparing phosphor composed of columnar crystals by employing CsI, as described in Patent Document 2. Such the scintillator plate is capable not only of controlling degradation of sharpness, but also of protecting the phosphor to be covered with the films.
(Patent Document 1) Japanese Patent O.P.I. Publication No. 2003-66147
(Patent Document 2) Japanese Patent O.P.I. Publication No. 63-215987